As known in the art, a DC brushless motor is similar in performance and application to a brush-type DC motor. Both have a speed vs. torque curve, which is linear or nearly linear. The motors differ, however, in construction and method of commutation. A brush type permanent magnet DC motor usually consists of an outer permanent magnet field and an inner rotating armature windings to maintain rotation. A DC brushless motor has a wound stator, a permanent magnet rotor assembly and a sensing device to sense the rotating position. The sensing device provides signals for electronically switching (commutating) the stator windings in the proper sequence to maintain rotation of the magnet rotor.
In the family and industrial usage, a DC brushless motor combined with a compressor can be used in a DC variable-frequency air conditioner. Ordinarily speaking, the compressor is a single compressor or a twin compressor. Due to the structure of the single compressor, during one operating cycle of the single compressor, half of the operating cycle is used for drawing in the refrigerant while the other half of the process is used to compress the refrigerant. However, the pressure of the single compressor differs at drawing and compressing. Hence, from the view of the motor, the load of the motor varies with the rotational position. In other words, the load of the motor is asymmetric; for half of the operating cycle the motor encounters a smaller resistance but for the other half it encounters a bigger resistance. The irregularity of the load thus causes torque ripple in the controlling motor. Further, when the motor is driven by a fixed voltage, problems such as vibration and noise are resulted from the asymmetric load, particularly during slow rotation speed.
Therefore, a DC variable-frequency air conditioner uses the DC brushless motor combined with the more complicated twin compressor. Because the load of such kind of twin compressor does not vary with the rotational position of motor, discontinuity of the motor torque will not happen in the controlling motor.
However, due to the higher cost of the twin compressor, manufacturers would have difficulty in reducing production cost if twin compressor was used. Thus, how to control the DC brushless motor coupled with the low-cost single compressor while reducing torque ripple is the main object of the present invention.